Complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) can be utilized to convert optical images into corresponding electronic signals, and can be used as the sensing part in digital still image camera systems or digital video camera systems. The main advantage of CMOS APS compared to the conventional charge-coupled device (CCD) imager is that the CMOS APS can be fabricated in standard CMOS process. Therefore, the CMOS APS imager and other CMOS circuits can be integrated into a single chip. Consequently, the cost, power consumption, and complexity of the whole system can be greatly reduced.
In many digital camera applications, motion detection is a desirable function, and especially in security applications. The simplest way to detect motion is from the difference of two consecutive image frames. Large difference between two consecutive frame means there is a motion between the images. Therefore, if the image sensor is capable of direct frame difference readout, it can be used to detect motion.
Another main application of image sensors is the digital video camera. Due to the large amount of data in video signals, video compression is essential in limited transmission bandwidth and storage size. In video signals, two consecutive image frames tend to be very similar, because a short period of time elapses therebetween, and thus the amount of information in the frame difference is much less than that in the digital camera applications. If the frame difference between two consecutive image frames in video signals can be readout directly from the image sensor, video compression can be achieved by removing the temporal redundancy.
In order to obtain the frame difference from traditional image sensors, a frame buffer is needed to store the previous frame data, and then take the difference between the current frame data and the previous frame data. In this approach, the frame buffer will occupy a large amount of chip area. If the image sensor can provide direct frame difference output, the complexity and area of the chip can be reduced.
Traditional photo diode CMOS APS can not provide frame difference directly. Photo gate APS with intra pixel charge transfer has an operating mode for frame difference output. However, when the sensor is operating in this mode, the exposure time must be the frame period, which can not be easily changed. In real applications, the environment luminance change significantly, and it is essential to effectively control the exposure time to get good quality images.
Therefore, a CMOS APS image sensor with direct frame difference output for motion detection and video compression applications and with electronic shutter to control the exposure time is desirable.